Science Questions

How many genes do I share with grass?

Sat, 14th Jun 2014

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martin richards asked:

Given that all life on earth is supposed to be Descended from a common source how much of our DNA do we share with some thing like a blade of grass?



Harriet::  Listener Martin Richards asks, “Given that all life on earth is supposed to be descended from a common source, how many genes do I share with a blade of grass?  Here’s Dr Aylwyn Scally from the Department of Genetics at the University of Cambridge.

Aylwyn::  When we ask about shared genetic information between species, in other words, when we compared the DNA sequences or genomes of one species with another, we’re really asking about the common ancestry of those species.  So for example, with humans and chimpanzees, when we look at the chimpanzee genome, about 99 per cent of it matches up with some place in the human genome.  And that’s because chimpanzees are very close in evolutionary terms to us.  The common ancestor was about 6 million years ago – a long time for us, but short time for evolution. 

We go a bit further back to the common ancestor of say, humans and mice, that’s about 10 times longer ago.  There's been a lot more time for divergence and then we find only about 75 per cent.  About 75 per cent of the mouse genome can be matched up almost exactly with some area in human.  When it comes to comparing humans or any animal with a plant such as grasses, we’re then talking about a much, much greater gulf in time, around about 1.5 billion years.  As you might expect, there's very little that remained unchanged in the genomes of either lineage since that time.  Trying to make the same comparison that I just talked about, around about 25 per cent is representative thereabouts of the kind of similarity at that scale. 

However, we can relax the constraint somewhat.  We look for similarity which is not exact, but nevertheless, close enough that there's similarity in function between regions of the genome.  Then when we look at plant genes, we can find over half of them have some recognisable counterpart in humans.  And that’s presumably because those genes had an important role in the common ancestor and continue to do so in their descendants.

Kat:: Thanks to listener Martin Richards, Cambridge University’s Dr Aylwyn Scally and Harriet Johnson. And if you’ve got any questions about genes, DNA and genetics, just email them to me at


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martin richards asked the Naked Scientists: Given that all life on earth is supposed to be descended from a common source how much of our DNA do we share with some thing like a blade of grass? And what form of life on earth has the most complex DNA? What do you think? martin richards, Tue, 20th May 2008

Steve Jones, professor of genetics at UCL, told me that humans share about 60% of their genes with a banana. Given that grasses and bananas are both plants, the same figure seems like a reasonable estimate for a blade of grass.

Of course, when he says we share 60% of our genes, what he means is that the same genes are present in both bananas and humans i.e. pieces of genetic material that encode a protein that carries out a certain job in a cell. The actual genetic sequence of those genes will be subtley different between species, but the jobs they do will be grossly similar.

Chris chris, Wed, 21st May 2008

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